1. Field of the Invention
This invention relates to a method and apparatus for confirming that a microwave oscillator and balanced diode mixer combination is operating properly.
2. Description of the Prior Art
Microwave doppler type motion detectors are commonly used in connection with intruder alarms and automatic door opening systems. Such detectors operate by transmitting a microwave field within a volumetric space. They detect the presence of an intruder by receiving an echo signal, the frequency of which is shifted from the original frequency by an amount which is proportional to the relative velocity of the intruder. One type of receiver circuit which can be used for detecting the Doppler echo signal is a common balanced diode mixer/oscillator combination. Balanced diode mixers provide a simple and effective means for converting a high frequency microwave signals (RF) to an intermediate frequency (IF) doppler signal, which is more suitable for amplification and signal processing.
In order to operate, such balanced diode mixers generally require a microwave source to serve as a local oscillator. Various types of microwave sources, such as a Gunn diode or transistor-based microwave circuits, may be used for this purpose. However, over a period of time, the microwave source is prone to degradations in performance and failure. Such degradations in performance will have an adverse impact upon the operation of the mixer. Further, in Doppler type motion detectors, it is common to make use of a single microwave source to provide a local oscillator signal and a transmit signal. In such cases, the local oscillator signal may be derived by coupling a portion of the microwave transmit signal from the transmission line carrying the transmit signal to the transmitting antenna. If the power output level of the microwave source is low or if an antenna fault exists, the power output of the microwave source will most likely be decreased. This will also lead to a degradation in the performance of the motion detector. Thus, it is desirable to be able to periodically verify the performance of the microwave source.
Various methods are possible for verifying the performance of the microwave source in the foregoing types of systems. For example, means may be provided for coupling a portion of the RF signal generated by the microwave source to a detector so as to directly measure the RF output of the source. However, such systems have the disadvantage of being relatively expensive. As an alternative, some systems have developed a method of testing or verifying the performance of the microwave source by detecting an offset voltage, which is generated by an imbalance with respect to the diode pair which are used as part of the balanced mixer. This means of testing relies upon the fact that the diodes which are commonly used within the above-referenced balanced mixer circuit are often slightly mismatched with regard to their electrical characteristics. Thus, when the microwave source signal is applied from the local oscillator, a small offset voltage is generated. This offset signal can be as much as a few millivolts or less than a few microvolts, depending on how closely the two diodes are matched. If the signal is sufficiently large, it can be detected after passing through the amplifier circuitry, in a manner analogous to and at a signal level comparable to that of the doppler signal--which is the preferred condition.
Although the foregoing test method has proved acceptable in the past, a problem has arisen as a result of diode components which are more consistent with regard to their manufacturing tolerances. Specifically, because of improvements in the manufacturing for the commercially available diodes used in such balanced mixers, it has been found that their electrical characteristics have become more closely matched. Significantly, when such diodes are used in the context of a balanced mixer, the offset voltage, which had previously been relied upon to verify the operation of the microwave source, may no longer be present. In other words, the diodes may be so well matched that an offset signal is no longer present, or may be too near the noise threshold of amplifier circuits to be easily detected. While the output of the balanced mixer is typically coupled to an amplifier of relatively high gain in order to sense the doppler motion signal, the offset voltage may not produce an easily verified signal at the amplifier output if the offset signal is too small. It is not, moreover, usually desirable to increase the gain of the amplifier beyond a certain point for the purpose of detecting lower magnitudes of offset voltage, since to do so would add cost and could possibly cause interference with the dynamic range and signal to noise ratio of the amplifier. If the dynamic range or signal to noise ratio of the amplifier were adversely affected in this regard, it could conceivably interfere with the primary purpose of such circuitry, i.e., distinguishing doppler signals created by motion from spurious sources such as electrical noise.
Further, while the offset voltage method described above can be used to verify that the microwave source is operating, it does not provide a reliable means for determining if there has been any degradation in power output. At best, the foregoing method can only provide a binary result, i.e., the microwave source is working/not working. Also, if both diodes suffer a failure in the open state, no offset signal will be generated and confirmation of proper operation cannot be made.
Thus, it is an object of this invention to provide an improved method and apparatus for verifying the presence of a microwave source signal when the source is used in conjunction with a balanced mixer circuit. It is a further object of this invention to provide a method and apparatus for obtaining an approximate indication of the power output of the microwave source, so as to determine whether the source is operating within a desired range. It is still a further object of the invention to enable verification of an entire microwave receiver circuit i.e. a local oscillator, balanced mixer, IF amplifier and motion detection circuitry. Finally, it is also an object of the invention to create a repeatable test with known input parameters, whereby the test results of a current test may be compared with a previously stored test result.